The present invention is directed to a lens back focus controlling device which is used to control the magnification of an optical system in a microfilm copying machine or the like.
Microfilm copying machines for enlarging and copying a picture recorded on a microfilm are old and well known in the art. Most microfilm copying machines are so designed that a copying magnification is selected since the size of the picture recorded on a microfilm is considerably different from the size of the desired copy. In conventional copying machines, different lenses are provided for different magnifications and each lens specifically provided for a selected magnification is set at a predetermined position in the optical system to perform the copying operation. Therefore, as the number of desirable magnifications is increased the number of lenses to be prepared in advance must be increased accordingly. This substantially increases the manufacturing cost of the copying device. Even if a copying device has a large number of lenses, it is still impossible to fine tune or precisely change the set magnification and, accordingly, it is impossible in practice to achieve an enlarged picture of a precise desired size.
In order to overcome the aforementioned difficulties it has been proposed to vary the lens back focus in order to continuously vary the copying magnification. Such a prior art copying machine is shown in FIG. 1 wherein the light from a light source 10 is applied through a condenser lens 12 to an original microfilm copy 11. The light passed through the original microfilm copy 11 then passes through a lens 14 which is freely movable up and down and then is reflected in sequence from four mirrors 15, 16, 17 and 18 onto the surface of a photosensitive drum 19. The photosensitive drum 19 has been charged before an optical image is formed thereon. As the drum 19 is rotated an exposure process and a development process are successively carried out and a toner image is formed on the drum 19. The toner image is transferred onto a belt carried sheet which is conveyed along a path indicated by the dot-dash chain line 41. After being heated and fixed in the unit 42, the sheet is delivered into a discharge tray (not shown) thereby completing the copying operation.
The operator selects a copying magnification for the copying machine. A central processing unit or microcomputer provides instruction signals to move the lens 14 and the movable mirrors 16 and 17 predetermined distances in the directions of the arrows 13 and 44, respectively, according to the copying magnification thus selected. According to the instruction signals a drive mechanism (not shown) is operated to move the lens 14 and the movable mirror 16 and 17 so that the exposure process is carried out with the desired magnification. In the above-described copying machine the distance between the microfilm surface of the original microfilm copy 11 and the surface of the lens 14 must be adjusted precisely to the order of several microns whenever the copying magnification is changed. Therefore, even if a drive mechanism such as a screw engaged with a threaded hole in the mounting frame (not shown) of the lens is turned a predetermined number of revolutions by a pulse motor (not shown) to a position the lens 14, it is still impossible to adjust the distance with the desired precision since the precision of the screw thread is limited and may have some degree of play. Thus, a suitable driving arrangement for the lens 14 in order to achieve a very precise magnification is not available for use with a copying machine which is used to copy microfilms.